1. Field of the Invention
This invention relates to removal of local oxidation and corrosion products from portions of hardware without affecting adjacent, coated regions, and more particularly to removing local oxidation and corrosion product from airfoils removed from turbine service without affecting adjacent aluminide coatings.
2. Discussion of the Prior Art
Components such as turbine airfoils operate under strenuous environmental conditions at elevated temperatures. These components typically are coated with an aluminide as a bond coat or as an environmentally-protective coating. The harsh environment and elevated temperatures result in localized attack of the component that may penetrate the coating and work into the diffusion zone between the coating and the substrate. Repair of turbine hardware typically has involved removal of loose contamination followed by removal of tightly adherent corrosion and oxidation as the first steps in the repair process. This cleaning subjects the hardware to mechanical cleaning, such as abrasive cleaning or grit blasting, or to chemical cleaning. Chemical cleaning involves exposure of the hardware to chelating agents or immersion in high temperature caustic solutions. In order to complete the repair, the diffusion aluminide layer which was applied either as a bond coat or as an environmental coat, is removed by exposure to application of or immersion in an acid solution. Damage is then repaired, typically by welding, and a new aluminide coating is generated. The disadvantage of such methods is that the wall of the turbine hardware is affected by the cleaning and coating stripping processes since the protective aluminide coating is diffused into the original component wall and metal required to carry the load of operation is removed. Repeated stripping of the component thus typically limits the number of repair cycles that can be employed. Typically, only one stripping can be successfully accomplished due to concerns with loss of wall thickness.
While various methods exist for cleaning contaminants and oxides from the surfaces of components as part of the repair and restoration procedure, most of these methods involve subjecting the entire component to the cleaning operation, even when only a portion of the component requires cleaning and repair. One such method is set forth in U.S. Pat. No. 4,317,685 ('685 patent) to Ahuja et al. and assigned to the same assignee of the present invention. The '685 patent employs an aqueous solution of alkaline hydroxide. The component to be cleaned is placed in an autoclave with the aqueous alkaline hydroxide solution and heated to an elevated temperature in the range of 200-340.degree. C. The solution is permitted to react with the surface scale, and the reaction product is removed from the surface by flushing and the remaining process as described above is accomplished by removal of the remaining aluminide by acid etch, repair of the affected area and subsequent realuminiding, so that the problem of wall thinning is not addressed by this process.
An alternative approach for repairing turbine airfoil components that avoids the loss of material from load bearing walls is to apply an aluminum coating over the existing coating, thereby replenishing the protective aluminum and permitting further engine exposure. The impediment to this approach is the presence of oxidation and/or corrosion products on the surface of the hardware after removal of service in the turbine engine.
Some methods exist for cleaning corrosion from localized regions of a surface, but these are generally restricted to removal of trace metal contamination from surfaces of semiconductors. These methods would have no application to turbine components from which contamination and oxides must be removed, as semiconductors are generally an oxide species. One example of such cleaning is set forth in U.S. Pat. No. 5,695,570 to Douglas, that involves applying an ambient species to the contaminated surface, followed by photostimulation to allow reaction of the trace metal contamination with the ambient species with subsequent removal of the metal products from the surface.
What is needed is a method for accomplishing the repair of a turbine airfoil component by cleaning only the localized regions of the component affected by corrosion and oxidation without detrimentally altering adjacent regions of the coating unaffected by corrosion and oxidation, followed by repair of the locally cleaned region and application of an aluminide coating to the region of local repair.